1. Field of the Invention
The present invention relates to a jitter estimating apparatus and estimating method.
2. Description of the Related Art
A clock frequency of a microprocessor doubles every approximate 40 months. It is necessary to accurately measure jitter in a clock signal according to a shorter clock period. This is because a timing error is avoided in a system operation.
There are period jitter and timing jitter in jitter. For example, an operation frequency of a microprocessor in a computer is limited by period jitter in the clock signal in the microprocessor. Therefore, period jitter becomes a problem. Timing jitter becomes a problem as shift out of an ideal timing point in data communication.
FIGS. 1A to 1C illustrate jitter in the clock signal. In the ideal clock signal which does not include jitter, since an interval Tint between a prescribed rise edge of the ideal clock signal and a rise edge adjacent to the prescribed rise edge is constant as shown with a wave of a dotted line in FIG. 1A, period jitter is zero. A rise edge is wobbled before and after an arrow in an actual clock signal. Therefore, interval Tint is also wobbled with the wobbling of the rise edge. This wobbling becomes period jitter in the clock signal. Period jitter becomes a problem, for example, in the clock signal of the microprocessor in the computer.
As shown in FIG. 1B, in a case where an ideal pulse signal without jitter is waveform of a broken line, an edge of a pulse signal with jitter (solid line) and the edge of the ideal pulse signal (broken line) is shifted. This shift width is timing jitter.
A time interval analyzer or an oscilloscope is used as means of measuring the jitter. They measure jitter by a method called as a zero cross method.
FIG. 2 illustrates a conventional jitter estimating apparatus using the time interval analyzer. In the conventional jitter estimating apparatus, the time interval analyzer 12 receives a clock signal (tested signal) x(t) output from a tested PLL (phase-locked loop) 11. In the signal x(t), a next rise edge is wobbled against one rise edge as shown with a dotted line in FIG. 2. An interval Tp of both rise edges, that is, a period of the tested signal x(t) is wobbled. The time interval analyzer 12 measures a time interval between zero cross points of the signal x(t), that is, the period of the signal x(t). Histogram analysis for wobbling of the measured period is displayed.
FIG. 3 illustrates histogram of the period measured by the time interval analyzer. About the time interval analyzer, there is described in “Phase Digitizing Sharpens Timing Measurements”, by D. Chu (IEEE Spectrum, pp. 28–32, 1988), and “A Method of Serial Data Jitter Analysis Using One-Shot Time Interval Measurements” by J. Wilstrup (Proceeding of IEEE International Test Conference, pp. 819–823, 1998).
FIG. 4 illustrates a jitter estimating apparatus using a digital oscilloscope. FIG. 5 illustrates components of the jitter estimating apparatus in the digital oscilloscope 14. FIGS. 6A and 6B illustrate a tested signal and period jitter measured by the digital oscilloscope.
In recent years, a jitter estimating apparatus to measure jitter using an interpolation method is provided. A method of estimating jitter using the interpolation method (interpolation base jitter estimating method) is a method to measure timing of zero cross by interpolating between measured data close to zero cross in measured data of a sampled tested signal. That is, a time interval (period) between zero cross points is estimated by interpolating data and wobbling of the period is estimated.
The digital oscilloscope 14 receives the tested signal x(t) output from the tested PLL 11. In the digital oscilloscope 14, an A/D converter 15 converts the received tested signal x(t) into a digital signal. An interpolator 16 interpolates a signal value between values in which values of the digital signal is close to zero cross in the digital signal.
A period estimator 17 measures a time interval between zero cross and a histogram estimator 18 displays histogram of the measured value. An RMS and peak-to-peak detector 19 calculates a square mean and peak-to-peak value of wobbling of the measured time interval. In a case where the tested signal x(t) is a wave shown in FIG. 6A, period jitter is measured as shown in FIG. 6B.
It becomes a problem in an application of a computer for example whether or not the microprocessor normally operates even with a state where a worst value of period jitter in the clock signal of the microprocessor, an adjacent edge interval of the clock signal is maximum or minimum caused by the jitter. Based on this point, the quality of a microprocessor is judged by measuring the worst value, for example, of period jitter in the microprocessor and by judging whether or not the worst value is less than a prescribed value.
Especially, in a case of testing an electric device to generate a periodic signal such as a mass manufactured microprocessor, since it is necessary to measure jitter in a short time, the jitter estimating apparatus and the jitter estimating method capable of precisely measuring jitter in the short time are desired.
However, since there is dead time until next period measurement after a first period measurement in the conventional time interval analyzer, it takes time to obtain the number of data needed for histogram analysis. The digital oscilloscope cannot estimate histogram of jitter correctly and therefore jitter is over-evaluated.